All gases absorb light energy. For any particular gas, however, there is an associated absorption band for light energy known as that gas's signature band. A gas will absorb light energy having a wavelength that falls within its signature band. For example, carbon monoxide (CO) absorbs light with a wavelength of 4.6 .mu.m.
Semiconductor LEDs and laser emitters (hereinafter "emitters") can be fabricated to emit light energy at specific wavelengths. A gas having a signature band at one of those wavelengths will readily absorb the emitted light. Consequently, the intensity of the emitted light is inversely related to the concentration of gas through which the light has passed. Because of the relatively good gas absorption characteristics of infrared light, infrared emitters are used in certain gas detection systems. For example, infrared emitters operating in the 3-5 .mu.m wavelength range are used in conjunction with infrared sensing devices to detect and measure concentrations of noxious gases, such as carbon dioxide (CO.sub.2), carbon monoxide (CO), nitrous oxide (N.sub.2 O), trioxide (O.sub.3), or sulfur dioxide (SO.sub.2). In that regard, 3-5 .mu.m emitters have been very useful in environmental monitoring, medical diagnostics, and industrial process control applications.
Nevertheless, there have been numerous problems with existing infrared emitters. For example, although 3-5 .mu.m emitters have been formed from epitaxial films made of indium arsenic antimony phosphorous (InAsSbP) materials, substrate materials that could be lattice-matched to those films did not exist. Consequently, the performance of InAsSbP emitters has been somewhat limited.
Infrared emitters also have been formed from mercury cadmium telluride (HgCdTe) materials. The HgCdTe emitters can be lattice-matched to certain substrate materials. However, operating at the 3-5 .mu.m wavelength range, HgCdTe emitters had to be cooled with refrigerants or cryogenics equipment, since they functioned properly only at temperatures much lower than 100.degree. K. Such cooling equipment is relatively bulky and expensive, which has increased the space requirements and cost of previous 3-5 .mu.m HgCdTe emitters.
Accordingly, a need exists for a 3-5 .mu.m emitter, which can be lattice-matched to a substrate material and can also operate at ambient or room temperatures.